The present invention relates to an improved method and apparatus for extrusion molding of thermosetting resins.
Methods available for molding thermosetting resins are compression molding, transfer molding, injection molding and extrusion molding. For extruding simple long profiles such as round bars and pipes, plunger type molding equipment is commonly employed. However, plunger machines have a low productivity and involve difficulty in producing extrudates of consistent quality because they provide high extrusion pressure in the mold cavity and can achieve only intermittent extrusion cycles. In order to eliminate these defects, extrusion molding using screw-type equipment has been developed. According to this method, a melt of thermosetting resin in the screw machine is introduced through an adapter into the mold where it is given the final profile. However, it is still impossibly difficult to achieve continuous and consistent molding by this method. As shown in Japanese Patent Application (OPI) No. 146860/79, the screw-type equipment has complex resin channels. Thus, slight fluctuation in temperature or pressure cause a rapid progress of the thermosetting reaction or stagnation of the melt can occur, which causes a localized setting reaction. Therefore, much effort has been made to solve these problems with the screw-type extrusion machines.
Japanese Patent Application (OPI) No. 128521/82 shows a screw-type machine for the extrusion of cross-linked polyethylene. In order to prevent deterioration of the weld strength due to torpedo fins, a torpedo whose outside diameter is 3/10 to 4/10 of the screw diameter and which penetrates through the screw is coupled to the drive mechanism of the extruder and is rotated for orienting the resin feed. However, even this screw type machine does not suggest continuous extrusion molding equipment suitable for the molding of thermosetting resins because the machine involves structural limitations on the outside diameter of the torpedo, namely, the inside diameter of a designable resins pipe.